The manufacture and use of improvised explosive devices (IEDs) presents a serious hazard to military and civilian personnel in conflict zones throughout the world. Consequently, it is critically important to be able to identify and dismantle manufacturing sites for these devices before they can be fabricated and deployed. A key element in this strategy is the detection of the presence of residual explosive components at or near the sites, providing confirmation of illicit activity and allowing authorities to coordinate efforts to identify and detain persons associated with IED manufacture. As current detection methods for readily available nitrogen-based explosives continue to evolve in terms of sensitivity and selectivity,1-5 however, enemy combatants are increasingly turning to alternate explosives based on chlorate or peroxides, for which fewer means of reliable detection in the field are available.
In particular, chlorate detection is complicated by a complex pH-dependent redox chemistry that facilitates (inter)conversion of chlorate to other chlorine-containing species such as hypochlorite, chlorite, chlorine dioxide, chlorine, and chloride.6-8 Ion exchange chromatography provides a means for rapid separation of such Cl-containing species and, coupled with sensitive mass spectrometric detection, permits quantitative determination of each species in a laboratory setting.9,10 However, carrying out such an analysis in the field, where rapid determinations must be made under often adverse conditions, presents serious logistical issues, especially in terms of safety, weight, and power requirements.
Simpler spectrophotometric methods for chlorate detection are, in theory, amenable for field use. These methods are generally based on bleaching of the color of a dye species6,11-14 or the catalyzed generation of colored triiodide anion15-18 in the presence of chlorate. The color change may be visible to the naked eye or with the aid of very simple instrumentation, permitting development of a lightweight system that requires little or no power to operate. However, specificity for chlorate detection generally remains an issue, since various other chlorine species such as hypochlorite, chlorite, chlorine, and chlorine dioxide are also strong oxidants capable of interfering and rendering a false positive signal.
Electrochemical methods that exploit the redox behavior of the chlorate species provide a convenient alternative to spectrophotometric detection methods. While electrochemical methods do require a power source, recent advances in electronics miniaturization and design can now provide lightweight, low-cost, rugged, low power potentiostats19 that limit the impact of this issue. Because a simpler “yes or no” determination, rather than a quantitative chlorate analysis, may be sufficient for on-site testing in the field, any sensitivity issues related to the use of these simpler instruments are less of a concern. There are, however, several other issues associated with electrochemical detection of chlorate that must be addressed, especially for field work.
First among these is electrode type. Previous systems for analysis of chlorate were based primarily upon polarographic and related techniques using Hg electrodes,20-25 which are not ideal for field use. During the 1990s, however, Gao and coworkers26-30 extended earlier work by Unoura31 and others32,33 demonstrating catalytic electroreduction of chlorate by polyoxometalates and related transition metal compounds in solution at Pt and glassy carbon electrodes by developing electrocatalytic carbon paste electrodes impregnated with carboxylate ligand species and polyoxometalates as chlorate sensors. More recently, Jakmunee and coworkers34 demonstrated amperometric detection of chlorate using a triiodide based scheme with stopped-flow injection, which was subsequently used for successful detection of chlorate in soil samples.35 
A second issue is the potential interference due to electrochemical signatures of other chlorine-containing species, such as hypochlorite, chlorite, and chlorine dioxide, which may be present with chlorate or generated from it during the course of sampling and analysis. Significant efforts and progress have been made to address this concern. For example, rather than detect chlorate directly, Wen and coworkers36 utilize it to selectively oxidize chalcopyrite (i.e., CuFeS2) and electrochemically detect the Cu(II) and Fe(III) released. Similar strategies have been demonstrated using sphalerite (i.e., ZnS2, producing Zn(II))37 and galena (i.e., PbS, producing Pb(II))38 as the metal ion sources. Elimination of Cl-containing interferences can also be accomplished via their preferential removal from a sample by reaction with N2H5+/OsO439 BH4−,40 or Fe(II)6 and/or careful adjustment of the reaction conditions41,42 prior to initiating the electrochemical analysis of chlorate. Finally, recent work indicates that selective surface modification of the electrode with rare earth coatings43,44 can hinder the reduction of certain Cl-containing species, such as hypochlorite, in the presence of chlorate.
Despite these advancements, the electrochemical analysis of chlorate under ambient conditions in the field remains hindered by the presence of oxygen, whose reduction (Epc>−0.3 V. vs. Ag/AgCl) is sufficiently close to that of chlorate (Epc≅−0.4 V. vs. Ag/AgCl) to interfere with the analysis. Although the pH dependence of the oxygen reduction potential can be exploited to lessen this effect, it cannot be entirely removed. In similar fashion, any attempt to deconvolute measured current data to account for the oxygen contribution requires simultaneous measurement of the oxygen level and introduces additional complexity and error sources into the analysis. Removal of oxygen from the sample by purging with inert gas can certainly solve the problem, but at the expense of longer analysis times and added inert gas container weight, both of which are problematic for field use.
G. Cao et al.45 described a technique wherein sulfur-polyoxometalate (POM) is mixed with methylene blue dye into a carbon paste, which is cast as an electrode. In the presence of 1M H2SO4 (strongly acidic conditions), this electrode could detect chlorate, however the electrode also exhibited significant instability. It is not apparent that this electrode could operate under, non-acidified conditions nor does not seem as if the electrode would have usefully long life in the field, and it must regularly by “renewed” by squeezing out fresh carbon paste containing the POM.
A need exists for an electrode that senses chlorate directly under ambient conditions from real world samples, such as soil, that may also be contaminated with traces of other electroactive species, such as humates, metal ions, and nitrogen-based explosives, among others.